1. Field of the Invention
The present invention is broadly concerned with cooking extruders of the type used for processing materials into animal feeds or human food products. More particularly, it is directed to such cooking extruders which are specifically designed to permit incorporation of very high quantities of steam into materials being processed, which allows the extruders to successfully process feed mixtures containing inexpensive ingredients such as rice bran with less mechanical energy and shear being required.
2. Description of the Prior Art
Extrusion cooking devices are used in a multitude of contexts, e.g., for the fabrication of animal feeds and human food products. Generally speaking, single screw extruders include an elongated barrel having an inlet at one end and an outlet at the other equipped with a restricted orifice die. An elongated, flighted, axially rotatable screw is positioned within the barrel and serves to move material from the inlet toward and through the outlet. Twin screw extruders are also widely used, and include within the extruder barrel a pair of side-by-side, flighted, intermeshed screws. All such extruder devices serve to cook and form initial starting materials into final extruded products. During the course of extrusion the starting materials are subjected to increasing levels of pressure and shear, in order to produce the desired, fully cooked, final extruded products.
In order to achieve higher levels of cook (commonly measured by the degree of gelatinization of starch-bearing ingredients and/or the level of denaturation of proteinacous ingredients), it is common to inject high pressure steam into the extruder barrel for incorporation into the materials being processed. For this purpose, steam injection ports are formed in the extruder barrel and communicate with the interior thereof. The ports are designed to house conventional steam injectors, coupled with steam lines. Without known exception, the injection ports in prior cooking extruders have been oriented in an orthogonal relationship relative to the barrel interior, or perpendicular to the longitudinal axis of the extruder screw(s). However, the extent of possible steam injection with conventional extruder designs is somewhat limited. That is, only about 3-5% by weight steam can be successfully injected and incorporated into the material being processed. If excess steam is injected, it tends to pass directly along the length of the extruder and out the extruder inlet opening (and sometimes the outlet die) without being incorporated into the material being extruded. As such, addition of excess steam serves no useful purpose.
It is known that steam injection to achieve higher levels of cook, thus avoiding the necessity of excess pressure, shear and mechanical working of the material being processed, can be highly advantageous. For example, some aquatic feed products are sensitive to high levels of pressure, shear and mechanical energy, and thus can be adversely affected using conventional extruders. Additionally, these feeds sometimes make use of relatively inexpensive ingredients such as rice bran, which are best processed using high steam injection levels.
Accordingly, there is a real need in the art for improved cooking extruder devices which can be used to inject greater quantities of steam into the material being processed, as compared with conventional extruder designs.